With the development of society and the progress of science technology, people increase the requirement to mobile communication continuously, data transmission service with large capacity, high speed, and low delay is desirable. TD-SCDMA system is the only one to adopt TDD method among the three mainstream standards in the third generation mobile communication system, the characteristic of TD-SCDMA is that uplink and downlink are in the same frequency band, a pair of frequencies are not necessary, the spectrum can be used with a relatively large flexibility. Furthermore, uplink and downlink can be flexibly configured to support the transmission of uplink and downlink asymmetric service. TD-SCDMA adopts advanced technologies in mobile communication such as smart antenna, uplink synchronization, joint detection, and software defined radio and the like to provide relatively high performance and spectrum utilization efficiency of the system.
From the frame structure in the current TD-SCDMA system shown in FIG. 1 and FIG. 2, the frame structure in TD-SCDMA system adopts three layers structure: radio frame, sub frame and time slot. The length of each radio frame is 10 ms, divided as two 5 ms sub frames; each sub frame is composed of seven service time slots from time slot 0 to time slot 6 and three special time slots. The three special time slots are Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS) respectively. Wherein, DwPTS is mainly used to transmit system downlink synchronization information, GP is used to provide the time interval for switching from downlink time slot to uplink time slot, UpPTS is used to transmit uplink synchronization information of user access.
In the current TD-SCDMA system, each 5 ms radio sub frame has two uplink and downlink switching points, which are located between TS0 and TS1, and between TS3 and TS4 respectively shown in FIG. 2. Wherein, TS0 are fixedly used to transmit downlink information, TS1 are fixedly used to transmit uplink information. Therefore, the position of the first switching point is fixed between TS0 and TS1, used for switching from downlink time slot to uplink time slot. In order to obtain system uplink synchronization, a switching guard period GP is provided between uplink pilot time slot and downlink pilot time slot. Another switching point can be located between any two time slots from TS1 to TS6, the switching point is a switching point from uplink to downlink, a dedicated guard period is not necessary.
The time division duplex communication system has the communication process as following: a transmitting end encodes and modulates data according to the preset radio frame format, then the data are transmitted through radio frequency (RF), a receiving end receives RF signal, the RF signal is demodulated and decoded to obtain corresponding data, then receive acknowledgement information is transmitted to the opposite end through a channel, the transmitting end transmits the following data after receiving the acknowledgement information, by analogy, the transmitting of all data are fulfilled.
A sliding transmit window is commonly built in the transmitting end, in order to guarantee reliable and sequential transmission, the data packet from the transmitting end to the receiving end must be located within the transmit sliding window, after the transmitting end receives receive acknowledge information from the receiving end, the sliding window slides forward, the data newly entered into the window will be transmitted. If the transmitting end does not receive the acknowledgement information from the receiving end, and the data packets within transmit sliding window are all transmitted, then the transmitting end will wait for the acknowledgement information and does not transmit data any more. If waiting for the receive acknowledgement information is time out, the transmitting end will discard the packet. Time from the transmitting end transmitting data to the receiving end to the receiving end returning the receiving acknowledgement information is called transmission delay. Call throughout of the transmitting end is determined by the size of transmit sliding window in the transmitting end and the transmission delay, when the transmit sliding window is fixed, the size of call throughout is determined by transmission delay, the smaller is the transmission delay, the larger the call throughout is. In mobile communication, transmission delay is related with the length of radio data Transmission Time Interval (TTI) of air interface, the smaller is the unit length of TTI, the smaller the transmission delay is.
Meanwhile, for physical layer transmission, if the way of adaptive modulation and coding (AMC) and hybrid automatic repeat request (HARQ) are adopted, the smaller is TTI of transmission data, the shorter the delay of feedback channel quality in the receiving end is, so that the more the channel quality feedback from the receiving end approaches to the actual channel quality when transmitting, thus a higher call throughout can be achieved by adopting AMC. Meanwhile, if retransmitting is happened, since TTI is relative small, so that the retransmitting time of HARQ is reduced, and spectrum utilization efficiency is also improved. Currently for TD-SCDMA system, TTI uses frame (Release 4 version) or sub frame (HSDPA service in Release 5 version) as unit.
Next TD-SCDMA communication system is taken as an example to explain its work process. Except for TS0, the configuration between uplink service time slots and downlink service time slots is supposed to be 1:5, firstly UE builds downlink synchronization with the network side through receiving DwPTS time slot information, UE reads cell broadcasting information in TS0 to obtain corresponding configuration information. UE transmits uplink synchronization information to the network side through UpPTS time slot. After the network side receiving the uplink synchronization information of UE and building uplink synchronization, the network side transmits control information to UE, UE transmits acknowledgement information to the network side after receiving the control information. The network side allocates corresponding channel resource to UE, the network side and UE communicate through the allocated channel resource.
When the network side chooses TS2 of the nth sub frame to transmit data on TS2, UE needs to transmit receive acknowledgement information to the network side after UE receives data and processes the corresponding processing. Because the size of TTI is 5 ms, UE can not transmit acknowledgement information to the network side earlier than in TS1 time slot of the next sub frame, namely, N+1th sub frame, from TS2 time slot of Nth sub frame to TS1 of N+1th sub frame the delay is relatively large. Therefore, currently the waiting time of UE to transmit is too long, so that transmission delay is relatively long; call throughout of user will be influenced.